The present invention relates generally to the field of digital cameras, and more particularly, to a system and method for directing image light onto an array of optical sensors in a digital camera.
A digital camera 102, FIGS. 1 and 2, typically includes a lens system 106 for projecting and focusing the image of a subject onto the surface of an electronic sensor 116. Digital cameras are described in the following patents which are hereby incorporated by reference for all that is disclosed therein. U.S. Pat. Nos. 4,131,919, 4,420,773, and 4,541,010. The digital camera 102 may have a housing 104 with elements such as a display 112 to indicate the status of the digital camera 102, a button 110 which may be pushed to cause the digital camera 102 to take a picture, and a flash 114 to illuminate a subject. The electronic sensor 116 in a digital camera 102 comprises an area array sensor, i.e., a two-dimensional array of individual optical sensors, or pixels 126, 127, 129, 131, 133, etc., FIG. 12.
The image quality of a digital camera 102 is determined, in part, by the xe2x80x9cspacial resolution,xe2x80x9d or the number of pixels 126, etc., in the electronic sensor 116. It is also determined by the bit-depth and signal-to-noise ratio of the pixels 126, etc., or the ability of the pixels to measure and quantify the image light, e.g. 118, 120, and 122, incident upon it.
A pixel 126 may be constructed in various known ways. Generally a pixel 126 is constructed of a material which converts image light 120 into electrical signals, which can then be processed and stored in the circuitry of the digital camera 102. As best seen in FIG. 3, a pixel 126 contains a light sensitive region 128 and one or more non-light sensitive regions 130 and 132. The ratio of light sensitive, or active, regions 128 to non-light sensitive regions 130 and 132 is referred to as the fill factor. The light sensitive region 128 may comprise a portion of a silicon wafer 134, which is surrounded by support circuitry such as polysilicon gates 136, 138, 142, and 144, metal conductors, channel stops, light shields 140 and 146, etc, forming a pit 148. The image light 120 must travel down through the pit 148 to the bottom where the light sensitive region 128 is located.
As digital cameras 102 are designed with higher resolution, requiring more pixels 126, the pixel size must necessarily be smaller to keep the overall size and cost of the digital camera 102 down. However, it is more difficult to scale down the electronic support circuitry constituting the non-light sensitive regions 130 and 132 than it is to scale down the light-sensitive region 128. Therefore, as pixels (e.g., 126) become smaller, the fill factor becomes smaller, and the ratio of the sizes of light sensitive 128 and non-light sensitive regions 130 and 132 in the pixel 126 is reduced. In other words, if a pixel 126 is scaled down to half the size, the scaled down pixel is less than half as sensitive to light as the larger pixel would be.
Microlenses 166, 168, FIGS. 3 and 4, have been employed to increase the fill factor of very small pixels. A microlens e.g. 166 is a small lens with approximately the same area as the entire associated pixel 126, and may be formed with photolithographic processes. The microlens 166 is positioned above the pixel 126, gathering nearly all the image light 120 incident on the pixel 126 and directing it to the light sensitive region 128 of the pixel 126, as best shown by FIG. 3.
As a result of the non-linearity of fill factor versus size described above, the light sensitive region 128 at the bottom of the pit 148 grows relatively smaller as pixel size decreases, and the height 150 of the pit wall increases, reducing the acceptable angle of incidence (e.g., 162, FIG. 4) of the image light 120. If the image light 120 is at too great an angle of incidence 162, it will terminate on the wall of the pit, such as on the light shields 140 and 146, rather than making it down to the light sensitive region 128 at the bottom of the pit 148.
Referring now to FIGS. 2, 3, and 4, placing microlenses 168 over the center of a pixel 152 has the disadvantage of only working well near the center of the optical axis 108 of the digital camera""s 102 lens system 106. If a pixel (e.g. 152) is located at the periphery of the electronic sensor 116, remote from the optical axis 108 of the lens system 106, FIG. 4, the angle of incidence 162 of the image light 122 is larger than the angle of incidence 162 for pixels (e.g., 126) near the optical axis 108, FIG. 3. In the peripheral pixel location shown in FIG. 4, the image light 122 passes through the microlens 168 and is focused not on the light sensitive region 128, but on a non-light sensitive region 130 such as a light shield 140. As a result, such pixels (e.g., 152) near the periphery of the electronic sensor 116 detect less image light 122 than a more centered pixel and the image quality of the digital camera 102 is degraded.
Referring now to FIG. 5, one prior solution to the problem described above has been to shift the microlenses 264 and 268 at the periphery of the electronic sensor 216 in towards the optical axis 208, so that they are no longer centered over their respective pixels 224 and 252. The microlenses 264, 266, and 268 are shifted in towards the optical axis 208 as a function of distance of the corresponding pixel 224, 226, and 252 from the optical axis 208. For the pixels 226 near the optical axis 208, the corresponding microlenses 226 are not shifted or are not shifted very far towards the optical axis 208. For the pixels 224 and 252 farther out from the optical axis 208, the corresponding microlenses 264 and 268 are shifted a relatively larger distance towards the optical axis 208. The microlenses 264, 266, and 268 are placed so that the greatest possible amount of image light 218, 220, and 222 is focused and directed toward the light sensitive regions 228, 254, and 270.
This approach of shifting the microlenses 264, 266, and 268 has several disadvantages. First, the microlenses 264, 266, and 268 are less effective at focusing to a well defined spot at large angles of incidence 262. Second, the height 150 of the walls of the pits 148 limit the angle of incidence, e.g., 262, of the image light rays 218, 220, and 222 that allows the image light 218, 220, and 220 to reach the light sensitive regions 228, 254, and 270. A third problem arises when color filters 272, 274, and 276 are placed in the path of the image light 218, 220, and 222 in order to produce a color image. Since the image light rays 218 and 222 with a relatively high angle of incidence 262 pass through the color filters 272 and 276 at an angle, their path through the dye in the color filters is longer, thus they are more heavily filtered. This can result in undesirable color shifts from the center to the edges of the resulting images.
Another prior solution to the problems described above, illustrated in FIG. 6, is the use of a telecentric lens 378. Image light rays 318, 320, and 322 produced by a telecentric lens 378 are focused downward on the pixels 324, 326, and 352 at a consistent angle of incidence, independent of the original angles of incidence of the image light 318, 320, and 322 before passing through the telecentric lens 378. FIG. 6 illustrates how the image light 318, 320, and 322 is directed in paths which are substantially parallel to the optical axis 308. However, using a telecentric lens 378 in a digital camera 102 makes it much larger, more complex, and expensive. Typical telecentric lens designs for digital cameras 102 may have twice the length, diameter and cost as a comparable non-telecentric lens design for a conventional film camera.
A need therefore exists for a small, simple and inexpensive lens and electronic sensor system for a digital camera which can focus a substantial amount of image light on the light sensitive regions of the electronic sensor.
To assist in achieving the aforementioned needs, the inventor has devised a system and method for using a field lens in a digital camera to focus and direct image light toward the pixels in an electronic sensor.
An optical system for detecting image light in a digital camera having features of the present invention comprises an electronic sensor having an area array of pixels for detecting the image light. One or more field lenses are positioned adjacent or in front of the electronic sensor so as to be substantially in the path of the image light. The one or more field lenses straighten the image light and direct it onto the area array of pixels. If more than one field lenses are used, they are each aligned over the optical axis of the optical system, forming a compound lens.
The electronic sensor is mounted in the digital camera in the path of the image light perpendicularly to the optical axis of the optical system. The field lens generally has a substantially flat bottom surface mounted adjacent the electronic sensor, and a convex upper surface.
The field lens is preferably mounted to the electronic sensor as the cover to the electronic sensor, or alternatively, if the electronic sensor is provided with a cover, the field lens may be mounted above the cover of the electronic sensor or mounted directly to the cover of the electronic sensor.
The present invention may also comprise a method for reducing the angle of incidence of image light rays falling upon an electronic sensor in a digital camera, including providing a field lens to straighten the image light rays, and mounting the field lens over the electronic sensor so as to be substantially in the path of the image light rays.